The instant invention relates to backfill compositions for mines in general and, more particularly, to a new and useful mine backfill that has improved flow characteristics.
In order to protect underground excavations, such as mines, from collapse, fillers are introduced into the open voids of the mine. Backfilling of mined-out cavities improves the structural integrity of the mine. Various materials have been utilized to backfill mines. Rocks, sand, tailings, grout, cement, elastomeric materials and the like, have been used as backfill with varying amounts of structural and economic success.
The most common mining practice is to backfill with hydraulic fill. Hydraulic fill is a mixture of alluvial sand and/or mill tailings and a relatively small percentage of cement. The backfill procedure typically requires large quantities of water to transport the hydraulic fill through a pipeline system to various underground locations. This large quantity of excess water reduces the hydration action of the cement in the hydraulic slurry. Furthermore, excess water containing significant quantities of cement must be drained from the solids and pumped back to the surface.
Paste fill has been developed for use as an alternate backfill procedure. With paste fill, a properly sized material may be transported by gravity or pumped underground with minimum water content in the mix. Paste backfill procedures provide distinct advantages over hydraulic fills.
Firstly, a stronger backfill is produced with an equivalent amount of binder or cement. Secondly, the clean-up and water removal problems normally associated with hydraulic fills are minimal or absent. Whereas hydraulic fill is made from a material having a sufficiently coarse size distribution, paste fills have a sufficiently fine particle component to minimize porosity and produce the xe2x80x9cpastexe2x80x9d characteristic.
Work by the inventors at an operating mine has shown that high-density backfill materials ( greater than 75% by weight of solid materials in a solids/water mixture) may be unable to flow in a pipeline unless the fine particular distribution of the solids component is carefully controlled. A paper by R.K. McGeary, xe2x80x9cMechanical Packing of Spherical Particles,xe2x80x9d Journal of the American Ceramic Society, 44, pp. 513-522, 1961, discloses criterion for determination of the xe2x80x9cbestxe2x80x9d fine particle distribution for a sand mixture in terms of size ratios and composition of a 2, 3 or 4 component sand mixture. For example, a mixture of a coarse, fine and ultrafine sand mixture with size ratios of 316 to 38 to 7 requires 65% coarse, 24% fine and 11% ultrafine material to give the maximum packing factor.
Hydraulic or paste backfill in the mining industry is typically a sand mixture where the coarse material is approximately 1,000 microns in size, the fine material is 120 microns and the ultrafine material is roughly 20 microns in size. The difference between a hydraulic fill and a paste fill is the content of size 20 microns or less materials. To make a high density hydraulic fill that can bleed and therefore drain its water into the stope, the content of ultrafine material is deliberately kept to a minimum ( less than 5% by weight of the sand mixture). For a pastefill material, however, the ultrafine content is raised to 15-20% by weight. This high content of ultrafine material provides a large surface area which can bind the water in the sand/water mixture and provide a non-bleeding paste-like material. This paste or non-bleeding material then allows for a waterless environment in the stope with the elimination of draining and water pumping required by the high density hydraulic fill materials.
High density hydraulic material has a water content in the range of 25% by weight whereas, depending on the fine particle distribution of the paste/sand materials, its water content can range from 10 to 20% by weight. For a particular paste fill material, the range is narrowed to 1 to 2%, e.g., 79 to 80% solids content, for the material to remain a paste. Too little water forms a dry mix which cannot flow; too much water yields a high density hydraulic mixture. The tight control of water content and fine particle distribution in paste materials as well as the excessive water content of the high density hydraulic material can sometimes lead to plug formation in a pipeline when good quality control on these variables is not maintained.
In addition, since the strength of a backfill mixture is controlled to a first approximation by its binder-to-water (B/W) ratio, excessive water content yields lower ultimate strengths and higher binder contents are then required in order to achieve the minimum strength targets (50, 90, 150 psi [345, 621, 1034 kPa] after 3, 7, 28 days of curing). The need for a balance between good flowability and high strength properties in these high density hydraulic or paste backfill materials and the requirement of a suitable criterion linking the fine particle distribution to these strength and flow properties of these backfill materials has led to the discovery of the principles behind this criterion for xe2x80x9cbestxe2x80x9d fine particle distribution to achieve both high strength and flowability simultaneously. These principles form the basis of the present invention.
The mine backfill composition of the present invention includes a main component, such as a known mixture of coarse, fine and ultrafine materials in water, and a flow enhancing, superfine material component mixed with the main component. The superfine material component has a particle size of less than 1 micron and is present in an amount of about 0.3 to 1.5% by weight, or preferably 0.5 to 1% by weight, with 0.5% by weight having been found to produce the best rheology or non-plugging flow characteristics for a sodium bentonite superfine additive.
The superfine material component is a hydrophilic material that binds water and is preferably a clay, in particular, sodium bentonite.
In order to achieve good strength in a backfill sand mixture for any water content, it is necessary to have a good particle packing criterion. McGeary""s paper indicates such a criterion for this purpose. Furthermore, he has indicated that introducing a small amount of a fourth component of small size to the previously described 3 component mixture resulted in a solids mixture that could flow. This observation led the present inventors to study the effect of low content (1% or less) clay-like materials on the flow and strength of typical backfill mixtures.
Previous studies revealed that for a Bingham-like material to be formed in a sand backfill mixture and to have linear flow in a backfill pipeline distribution, it is necessary that the surface area of the sand material be on the order of 1500 cm2/gm. Sand mixtures with total surface area below this value do not exhibit Bingham flow and can cause phase separation between the sand and the water to occur with the resultant formation of sand plugs. The pastefill criterion of 15-25% material of size less than 20 microns has a surface area of about 500 cm2/gm which is well below this minimum surface area criterion. Hence, unless a significant fraction of small material (e.g., 5 microns or smaller in size) is included in this less than 20 micron ultrafines, sand plug formation may occur when trying to cause this type of sand material to flow.
According to the present invention, a fourth component, a clay-like material, with size in the range of 1.0 to 0.1 micron and surface area of about 200,000 cm2/gm (20 m2/gm), is added to the backfill mixture to provide a xe2x80x9csurface area bufferxe2x80x9d to the mixture. For example, 1% of a sodium bentonite clay material being added to a typical sand material for backfill, which has been made for high density hydraulic mixture, will provide a surface area of 2,000 cm2/gm in the total sand/water mixture. Smaller amounts of such a flow aid would be required for a paste fill material with larger amounts of ultrafines in the sand mixture (15-20% by weight) in order to achieve the surface area minimum requirement. Other alternative superfine or clay-like materials include kaolin, talc, mill tailings and slimes.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.